1. Field of the Invention
The present invention relates, in general, to printed circuit boards.
2. Description of the Related Art
A printed circuit board is a board made of non-conducting material, such as plastic, glass, ceramic, or some other dielectric on which chips and other electronics are mounted. A multilayer printed circuit board is a printed circuit board consisting of two or more layers of board material. Each separate layer has its own metallic tracings to provide electrical connections between various electronic components and to provide connections to other layers. The layers are laminated together to produce a single circuit board to which the components, such as integrated circuits, resistors, and capacitors are attached.
One common multilayer printed circuit board scenario is to have alternating layers, where a first layer is composed in all or part of conducting materials, a second layer is composed mostly of insulating material, and a third layer is composed in all or part of conducting materials, with portions of the conducting materials in the first and third layer connected by conducting materials interposed within channels within the second insulating layer. Such first, second, and third layers typically form part or all of an electric circuit. Many such alternating layers are often used to construct a final circuit.
With reference now to FIG. 1, shown is a perspective view of a grossly simplified example of alternating layers within an multilayer printed circuit board which form equivalent electric circuit 150. Depicted is that equivalent electric circuit 150 has a current loop 152 from driver 102 to receiver 106. Current loop 152 travels an electrically conductive path provided by multilayer printed circuit board structure 154. Multilayer printed circuit board structure 154 depicts a first layer (not shown) having driver 102, metallic trace 104, and receiver 106 which are contained within a first layer (not shown) of printed circuit board structure 154. Driver 102 is illustrated as electrically connected to metallic trace 104 (e.g., a copper trace). Metallic trace 104 is shown electrically connected to receiver 106. Receiver 106 is depicted as electrically connected to metallic wire 112. Metallic wire 112 is depicted as electrically connected at point 162 with conducting plane 116. Conducting plane 116 is illustrated as electrically connected at point 164 to metallic wire 118. Metallic wire 112 and metallic wire 118 are depicted as contained within a cylindrical channel hollowed out from a second insulating layer (not shown) of printed circuit board structure 154. For sake of illustration and coordination with equivalent electrical circuit 150, electrical current loop 152 is shown flowing from driver 102 to receiver 106 through metallic trace 104. Thereafter, electrical current loop 152 is shown flowing through metallic wire 112, metallic conducting plane 116, and metallic wire 118 back to driver 102.
Referring now to FIG. 2A, depicted is an isolated perspective view of metallic conducting plane 116 and metallic trace 104 of FIG. 1. As has been described, metallic wires 112, 118 (of FIG. 1A) respectively electrically connect with conducting plane 116 at points 162, 164 on conducting plane 116 which are shown relatively xe2x80x9cin linexe2x80x9d with metallic trace 104. Viewed from the perspective of conducting plane 116, when relatively high frequency alternating current (e.g., current with frequencies substantially in excess of 10 kHz) is flowing in current loop 152 (of FIG. 1), metallic wires 112, 118 (of FIG. 1A) are respectively sourcing and sinking current into points 162, 164 on conducting plane 116. Insofar as conducting plane 116 typically has relatively uniform characteristics, return current 160, flowing between point 162 to point 164, will tend to follow a path substantially underneath metallic trace 104, since for relatively high frequency alternating current the path underneath metallic trace 104 is the path of least impedance for reasons well-known to those in the art. The magnitude of return current 160 will be substantially the same as that of source current 159, since together source current 159 and return current 160 make up loop current 152. However, since conducting plane 116 is of greater width physical width than metallic trace 104, although the majority of return current 160 will attempt to flow under metallic trace 104, in actuality it will be distributed across width 170 of conducting plane 116 in a fashion illustrated by FIG. 2B.
With reference now to FIG. 2B, illustrated is an example of distribution 161 of return current 160 (of FIGS. 1 and 2A) on conducting plane 116. Shown is that the majority of return current 160 (of FIGS. 1 and 2A) is distributed, or flowing, through the portion of conducting plane 116 which lies substantially directly below metallic trace 104.
Referring again to FIG. 1, those skilled in the art will recognize that it is not necessary for metallic wires 112, 118 to be present in order for a return current to be present on conducting plane 116. That is, the mere presence of an alternating current in metallic trace 104 proximate to conducting plane 116 is sufficient to induce a return current such as return current 160 (although metallic trace 104 is shown in a plane above conducting layer 116, metallic trace 104 could be coplanar with conducting plane 116, as will be demonstrated in the detailed description). See e.g., M. Zahn, Electromagnetic Field Theory: A Problem Solving Approach 361-363 (1979). Furthermore, in point of fact, in an actual implementation it is likely that both current resulting from metallic wires 112, 118 and from magnetic induction will be present on conducting plane 116. However, for ease of description the discussion herein focuses on the current resulting from the presence of metallic wires 112, 118, although it is to be understood that in addition to or in the alternative to such current resulting from the presence of metallic wires connecting at points 162, 164, a return current can be present arising solely from the presence of alternating current within metallic trace 104, when metallic trace 104 is located proximate to conducting plane 116. This fact is to be borne in mind whenever discussion is made of any return current described in the present application.
Unfortunately, as printed circuit board densities have increased, the structure illustrated in FIGS. 1, 2A, and 2B is becoming less and less practicable. Instead, it is becoming common within the art for conducting plane 116 to be split into two pieces, for a variety of reasons. Splitting conducting plane 116 gives rise to a number of practical problems, a few of which will now be described.
Referring now to FIG. 3, shown is a modified version of multilayer printed circuit board structure 154, referred to as multilayer printed circuit board structure 300, which is structurally similar to printed circuit board structure 154 except that a conducting plane is shown broken into first metallic conducting part 302 and second metallic conducting part 304, thereby forming split conducting plane 316. Depicted is that first metallic conducting part 302 and second metallic conducting part 304 are separated by dielectric-filled moat 306 which is typically composed of a dielectric material. Shown is that metallic wires 112, 118 respectively connect with split conducting plane 316 at points 362, 364 on split conducting plane 316.
Those skilled in the art will recognize that even though there is no contiguous return path from receiver 106 to driver 102 through split conducting plane 316, driver 102 and receiver 106 will still function because second metallic conducting part 304 and first metallic conducting part 302 function as a sort of xe2x80x9cparallel-platexe2x80x9d capacitor. This fact is illustrated by loop current 352 in equivalent electric circuit 350, wherein first metallic conducting part 302 and second metallic conducting part 304 separated by dielectric-filled moat 306 is represented by capacitor 353. Further shown is that loop current 352 can be viewed as composed of a source current 359 and a return current 360, each having substantially equal magnitude (since they make up loop current 352) but different distributions within their respective current flow paths.
Those skilled in the art will recognize that there are government (e.g., the Federal Communications Commission in the United States) and industry electromagnetic compatibility (EMC) standards which set limits on the amount of electromagnetic radiation which may emanate from electrical systems having integrated circuits. As will be described in the detailed description, it has been discovered that split conducting plane 316 composed of first metallic conducting part 302, and dielectric-filled moat 306, and second metallic conducting part 304 of multilayer printed circuit board structure 300 tends to radiate a substantial amount of electromagnetic energy above and beyond that radiated by metallic conducing layer 116 of multilayer printed circuit board structure 300 (which for all practical purposes amounts to almost zero radiated electromagnetic energy compared to that radiated by the foregoing described structure of multilayer printed circuit board structure 300). Insofar as the government and industry standards are aggregative, it is desirable that each printed circuit board component radiate as little electromagnetic energy as possible, because the smallest increase in radiated energy can often make the difference between passing and failing compliance standards.
It is therefore apparent that a need exists to decrease the electromagnetic energy radiated from split metallic conductors carrying electrical energy, such as split conducting plane 316 composed of first metallic conducting part 302, dielectric-filled moat 306, and second metallic conducting part 304 of multilayer printed circuit board structure 300.
It has been discovered that an apparatus can be produced which will substantially decrease radiated emissions from split metallic conductors carrying electrical energy. The apparatus provides a split metallic conducting plane having a split formed by a dielectric-filled moat spanning a width of a side of a first metallic conducting part running substantially parallel to a side of a second metallic conducting part, with the moat structured such that the side of the first metallic part has at least two indentations and such that the side of the second metallic part has at least two indentations, and where a metallic trace is located proximate to the split metallic conducting plane.
The foregoing is a summary and thus contains, by necessity, simplifications, generalizations and omissions of detail; consequently, those skilled in the art will appreciate that the summary is illustrative only and is not intended to be in any way limiting. Other aspects, inventive features, and advantages of the present invention, as defined solely by the claims, will become apparent in the non-limiting detailed description set forth below.